In Cabin Pressure, First Officer Douglas Richardson gives a fictitious explanation of the name of the town, and then the crew try to figure out how to get 100 otters into their jet.
The excerpt is hilarious enough, but it's even funnier in the mashup below. Some clever person has edited together bits of various Sherlock episodes to match the dialog in the excerpt. (In one of the lines, Arthur says the non sequitur, "Yellow car." That's because in this episode - titled Ottery St. Mary - the three of them are driving to Ottery St. Mary to deliver a piano. And in addition to trying to stuff 100 otters in the jet, they are also playing the Yellow Car game. Whenever anyone sees a yellow car, he has to say, "Yellow car."
Benedict plays Martin Crieff, of course. Mark Gatiss as Moriarty is also Douglas Richardson, and Martin Freeman as Watson is also Arthur the dim steward.